The invention concerns a method for the production of an aromate concentrate suitable for use as blending component for fuel, from feed hydrocarbon mixtures displaying boiling range substantially between 40.degree. and 170.degree. C. and containing several aromates in addition to non-aromates. The feed hydrocarbon mixture, without previous separation into individual fractions, is subjected to an extractive distillation with the use of N-substituted morpholine, the substituents of which display no more than seven C-atoms, as selective solvent, with substantially all of the low-boiling non-aromates having a boiling range up to about 105.degree. C. and a majority of the higher-boiling non-aromates having a boiling range between about 105.degree. and 160.degree. C. being distilled off as raffinate from the top of the extractive distillation column, whereas the main amount of the aromates, as well as the residual non-aromates, together with the employed solvent, being discharged as extract from the sump of the extractive distillation column, whereupon the hydrocarbons in the extract are distillatively separated from the solvent in a subsequently disposed solvent separation column and employed in whole or in part as blending component, while the solvent is returned to the extractive distillation column.
A method of this general type is known from German Offenlegungsschrift DE-OS 36 12 384, employing aromate-containing hydrocarbon mixtures as feed hydrocarbon mixtures. Particularly suitable for this purpose are reformate and platformate with not too high a content of benzene, from the working up of petroleum. However, mixtures of such reformate and platformate with pyrolysis benzene can also be employed.
With these entry products, the boiling limit indeed normally lies at 170.degree. C. However, it has turned out in practice that this boiling limit is not maintained in many cases, since the initial production processes result in a formation of condensation and polymerization products which display a higher boiling point than 170.degree. C., and which correspondingly contaminate the reformate and platformate. Thus, for example, a typical reformate from the working-up of petroleum displays a portion of higher-boiling components with a boiling point greater than 170.degree. C., to an extent of about 3% by weight. The composition of this higher boiling fraction is as follows:
______________________________________ Compound KP.degree.C. % by weight ______________________________________ m-cymol 175 3.4 hemmellitol 176.1 14.3 p-cymol 177.1 12.3 N-butylbenzene 183 2.8 indane 177.8 9.9 1,2-diethylbenzene 183.4 24.3 durene 196.8 4.7 I-durene 198 16.2 tetralin 207.6 0.1 trimethylethylbenzene 213 3.0 naphthalene 218 4.0 methyltetralin 229 0.1 .beta.-methylnaphthalene 241 2.0 .alpha.-methylnaphthalene 245 1.2 diphenyl 255 0.8 dimethylnaphthalene 268 0.9 TOTAL = 100.0 ______________________________________
Since the portion of these higher-boiling condensation and polymerization products, which shall be designated hereafter as heavy aromates, can amount in individual cases to substantially more than 3% by weight in the reformate and platformate, there can result difficulties during the performance of the method according to DE-OS 36 12 384.
It has been proven in practice that these heavy aromates become concentrated in the selective solvent. With progressive operational periods, this leads to ever stronger contamination of the solvent led in circulation, so that its selectivity is steadily decreased and the separation effect in the extractive distillation is correspondingly diminished. Attempts to separate out the heavy aromates by distillation of the solvent have provided no satisfactory results, even with high distillation expenditures, since part of the heavy aromate fraction boils in the same temperature range as the solvent. Inasmuch as a distillative separation is practically impossible, this problem could only be solved previously by providing a complete exchange of fresh solvent for the contaminated solvent after a certain operational period. Obviously, this technique is extremely costly, and thereby not economical. In addition, destruction of the contaminated solvent results in further cost, since it cannot be introduced to any other use or purpose.